The invention relates generally to powder metallurgy and, more particularly, to the application of mechanical alloying techniques to chemical refining through solid state reactions.
Most of the metals and metalloids of the periodic table form binary compounds of carbon called carbides. High temperature carbide ceramics, formed by consolidation of carbide powders, exhibit attractive characteristics including high strength, hardness, wear resistance, elevated temperature capacity and other properties that make them useful for a variety of applications. Applications for these ceramics are increasing due to their superior properties compared to alternate metallic and other non-metallic candidate materials. Some examples of application include cutting tools, mechanical seals, bullet proof vests, metal reinforcements, abrasive wear-resistant nozzles, bearings, grinding media for fine grinding of ceramic powders, tools and die components, heat exchangers, valves, igniters, pump components used in molten metal handling, furnace components in semiconductor manufacturing, electronic resistors, and laser mirrors. The physical and chemical properties of carbides are dependent on the nature of the metal/carbon bonds in these compounds. Depending on the nature of chemical bonds the carbides can be categorized into the following types:
(i) Ionic carbides: These are salt like carbides of metallic elements of groups IA, IIA and IIIB of the Periodic Table, the lanthanides and actinides included, e.g., CaC2, Na2C2, HgC2, LaC2, UC2, ThC2.
(ii) Interstitial carbides: These are metal-like carbides of the transition elements of groups IV, V and VI of the Periodic Table. The metallic character of these carbides is shown in their metallic luster, and high thermal and electrical conductivity. e.g. TiC, Nb2C, WC, MoC, TaC, Fe3C, Co2C.
(iii) Covalent carbides: These are diamond-like carbides having extreme hardness, which is exceeded only by diamond itself, e.g., B4C, SiC. Ionic carbides are highly reactive compared to other carbides. The interstitial and covalent carbides have high hardness and elevated melting points and their consolidation into carbide ceramics necessitates high temperature diffusion between the particles. Diffusion is easier for particles with larger surface areas. For this reason carbides are consolidated after comminution of commercially available powders to reduced sizes.
Development of cost effective processes and materials will greatly expand current uses and will lead to many new applications for the carbide ceramics. Currently, limiting factors in the wide spread use of carbide ceramics are the high cost of synthesis of powders and their consolidation, and the poor toughness of the finished material. These problems can be minimized if carbides can be synthesized in ultrafine powder form because materials with grain size in the nanoscale range possess unique physical and mechanical properties. Toughness of the carbides has been improved in the past by alloying different carbides to form solid solutions. However, work on formation of carbide solid solutions has progressed very little in the past, especially for ultrafine powders.
Conventional methods of production of ultrafine carbides are based on the reduction of a metallic oxide or compound by carbon or a carbon-containing compound at high temperatures followed by comminution.
Use of alloy carbides in place of single carbides reduces brittleness. A major break through in the manufacture of carbide ceramics occurred with the development of WCxe2x80x94TiC and WCxe2x80x94TaC alloy carbides in the 1930s. Generally duplex structures in cemented carbides are harder and tougher than single, unalloyed carbides. For example, the ductility of vanadium carbide and titanium carbide can be improved by alloying with other carbides. A resurgence of interest in alloys of carbides took place in the 1970s with the development of a series of patents on WC-free alloys including TiCxe2x80x94Mo2Cxe2x80x94Ni and (Ti,Mo,V,Nb)C1xe2x88x92x, and carbonitrides Ti(CN) and (Ti,Mo)(CN) for cutting tool applications.
The lack of availability of the ceramic carbide alloys relates to the problems associated with the synthesis of these powders. Conventionally, alloying of carbides has been pursued in different ways. The first, alloying of different carbide powders by solid state diffusion, second, carburization of metal oxide mixtures or metal powder mixtures with carbon black and the third, separation of solid solutions of carbides from metal melts. Despite the high temperature (1400-2600xc2x0 C.) of the process, it is difficult to retain high mutual solubility of the carbides because of the steep decrease in solubility with decreasing temperature. In the case of the Wxe2x80x94Taxe2x80x94C system, it is almost impossible to synthesize single-phase (W,Ta)C because of the rapidly decreasing solid solubility of WC in TaC, as the temperature is reduced. For this reason (W, Ta)C alloy is called a xe2x80x9cdouble phasexe2x80x9d carbide. These problems with single-phase production increase as the number of carbide forming elements in the material increase.
With the exception of SiC, carbide ceramics have not developed commercially to levels similar to those of oxide ceramics because synthesis of carbides, even at the micron sized particle level, is difficult because of the high process temperatures necessary. The carbide industry got a boost in the early 1970s with the recognition that carbides are potential materials for use in engines operating at elevated temperature. However, growth of the carbide industry has been limited by the unavailability of carbides in fine powder form at an affordable cost. Crushing and grinding of carbides can be used to produce 1-10 micron sized powder; but carbides are then expensive. The typical cost of sub-micron size titanium carbide, for example, varies from $30 to $100 per kg, depending on quality. The cost of production increases in proportion to the reduction in particle size. The availability of alloy carbides is even worse because of the necessity for additional processing steps. Thus there is strong technical xe2x80x9cpullxe2x80x9d for production of cost-affordable single carbides and alloy carbides.
The present invention is directed to the formation of metal and metalloid carbides by mechanically inducing a reduction reaction between a metal chloride (or a metalloid chloride) and a metal carbide. In the preferred embodiment of the invention, the reduction reactions are induced mechanically by milling the reactants. Group IA, IIA and IIIB carbides along with aluminum carbide and zinc carbide should provide suitable reducing agents. Alloy carbides may also be produced by mechanically inducing the co-reduction of metal chlorides or metalloid chlorides and a metal carbide according to the equation: M1chloride+M2chloride+M3carbidexe2x86x92M1M2carbide, where M1 is a metal or metalloid, M2 is a metal or metalloid and M3 is a suitable metal carbide reducing agent.